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Internet Engineering Task Force (IETF) D. Smith
Request for Comments: 6178 J. Mullooly
Updates: 3031 Cisco Systems
Category: Standards Track W. Jaeger
ISSN: 2070-1721 AT&T
T. Scholl
nLayer Communications
March 2011
Label Edge Router Forwarding of IPv4 Option Packets
Abstract
This document specifies how Label Edge Routers (LERs) should behave
when determining whether to MPLS encapsulate an IPv4 packet with
header options. Lack of a formal standard has resulted in different
LER forwarding behaviors for IPv4 packets with header options despite
being associated with a prefix-based Forwarding Equivalence Class
(FEC). IPv4 option packets that belong to a prefix-based FEC, yet
are forwarded into an IPv4/MPLS network without being MPLS-
encapsulated, present a security risk against the MPLS
infrastructure. Further, LERs that are unable to MPLS encapsulate
IPv4 packets with header options cannot operate in certain MPLS
environments. While this newly defined LER behavior is mandatory to
implement, it is optional to invoke.
Status of This Memo
This is an Internet Standards Track document.
This document is a product of the Internet Engineering Task Force
(IETF). It represents the consensus of the IETF community. It has
received public review and has been approved for publication by the
Internet Engineering Steering Group (IESG). Further information on
Internet Standards is available in Section 2 of RFC 5741.
Information about the current status of this document, any errata,
and how to provide feedback on it may be obtained at
http://www.rfc-editor.org/info/rfc6178.
Copyright Notice
Copyright (c) 2011 IETF Trust and the persons identified as the
document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents
(http://trustee.ietf.org/license-info) in effect on the date of
publication of this document. Please review these documents
carefully, as they describe your rights and restrictions with respect
to this document. Code Components extracted from this document must
include Simplified BSD License text as described in Section 4.e of
the Trust Legal Provisions and are provided without warranty as
described in the Simplified BSD License.
Table of Contents
1. Motivation ......................................................2
2. Introduction ....................................................2
3. Specification of Requirements ...................................4
4. Ingress Label Edge Router Requirement ...........................4
5. Security Considerations .........................................5
5.1. IPv4 Option Packets That Bypass MPLS Encapsulation .........5
5.2. Router Alert Label Imposition ..............................7
6. Acknowledgements ................................................7
7. References ......................................................7
7.1. Normative References .......................................7
7.2. Informative References .....................................8
1. Motivation
This document is motivated by the need to formalize MPLS
encapsulation processing of IPv4 packets with header options in order
to mitigate the existing risks of IPv4 options-based security attacks
against MPLS infrastructures. We believe that this document adds
details that have not been fully addressed in [RFC3031] and
[RFC3032], and that the methods presented in this document update
[RFC3031] as well as complement [RFC3270], [RFC3443], and [RFC4950].
2. Introduction
The IPv4 packet header provides for various IPv4 options as
originally specified in [RFC791]. IPv4 header options are used to
enable control functions within the IPv4 data forwarding plane that
are required in some specific situations but not necessary for most
common IPv4 communications. Typical IPv4 header options include
provisions for timestamps, security, and special routing. Example
IPv4 header options and applications include but are not limited to
the following:
o Strict and Loose Source Route Options: Used to IPv4 route the
IPv4 packet based on information supplied by the source.
o Record Route Option: Used to trace the route an IPv4 packet
takes.
o Router Alert Option: Indicates to downstream IPv4 routers to
examine these IPv4 packets more closely.
The list of current IPv4 header options can be accessed at [IANA].
IPv4 packets may or may not use IPv4 header options (they are
optional), but IPv4 header option handling mechanisms must be
implemented by all IPv4 protocol stacks (hosts and routers). Each
IPv4 header option has distinct header fields and lengths. IPv4
options extend the IPv4 packet header length beyond the minimum of 20
octets. As a result, IPv4 packets received with header options are
typically handled as exceptions and in a less efficient manner due to
their variable length and complex processing requirements. For
example, many router implementations punt such IPv4 option packets
from the hardware forwarding (fast) path into the software forwarding
(slow) path causing high CPU utilization. Even when the forwarding
plane can parse a variable-length header, it may still need to punt
to the control plane because the forwarding plane may not have the
clock cycles or intelligence required to process the header option.
Multi-Protocol Label Switching (MPLS) [RFC3031] is a technology in
which packets associated with a prefix-based Forwarding Equivalence
Class (FEC) are encapsulated with a label stack and then switched
along a label switched path (LSP) by a sequence of label switch
routers (LSRs). These intermediate LSRs do not generally perform any
processing of the IPv4 header as packets are forwarded. (There are
some exceptions to this rule, such as ICMP processing and LSP ping,
as described in [RFC3032] and [RFC4379], respectively.) Many MPLS
deployments rely on LSRs to provide layer 3 transparency much like
ATM switches are transparent at layer 2. Such deployments often
minimize the IPv4 routing information (e.g., no BGP transit routes)
carried by LSRs since it is not necessary for MPLS forwarding of
transit packets.
Even though MPLS encapsulation seems to offer a viable solution to
provide layer 3 transparency, there is currently no formal standard
for MPLS encapsulation of IPv4 packets with header options that
belong to a prefix-based FEC. Lack of a formal standard has resulted
in inconsistent forwarding behaviors by ingress Label Edge Routers
(LERs). When IPv4 packets are MPLS encapsulated by an ingress LER,
for example, the IPv4 header including option fields of transit
packets are not acted upon by downstream LSRs that forward based on
the MPLS label(s). Conversely, when a packet is IPv4 forwarded by an
ingress LER two undesirable behaviors can result. First, a
downstream LSR may not have sufficient IPv4 routing information to
forward the packet resulting in packet loss. Second, downstream LSRs
must apply IPv4 forwarding rules that may expose them to IPv4
security attacks.
IPv4 option packets that belong to a prefix-based FEC, yet are
forwarded into an IPv4/MPLS network without being MPLS-encapsulated,
present a security risk against the MPLS infrastructure. Further,
LERs that are unable to MPLS encapsulate IPv4 packets with header
options cannot operate as an LER in certain MPLS environments. This
new requirement will reduce the risk of IPv4 options-based security
attacks against LSRs as well as assist LER operation across MPLS
networks that minimize the IPv4 routing information (e.g., no BGP
transit routes) carried by LSRs.
3. Specification of Requirements
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in [RFC2119].
4. Ingress Label Edge Router Requirement
An ingress LER MUST implement the following policy:
o When determining whether to push an MPLS label stack onto an
IPv4 packet, the determination is made without considering any
IPv4 options that may be carried in the IPv4 packet header.
Further, the label values that appear in the label stack are
determined without considering any such IPv4 options.
This policy MAY be configurable on an ingress LER, however, it SHOULD
be enabled by default. When processing of signaling messages or data
packets with more specific forwarding rules is enabled, this policy
SHOULD NOT alter the specific processing rules. This applies to, but
is not limited to, Resource Reservation Protocol (RSVP) as per
[RFC2205], source routing as per [RFC791], as well as other FEC
elements defined by future specifications. Further, how an ingress
LER processes the IPv4 header options of packets before MPLS
encapsulation is out of scope since these are processed before they
enter the MPLS domain.
Implementation of the above policy prevents IPv4 packets that belong
to a prefix-based FEC from bypassing MPLS encapsulation due to header
options. The policy also prevents specific option types such as
Router Alert (option value 148) from forcing MPLS imposition of the
MPLS Router Alert Label (label value 1) at ingress LERs. Without
this policy, the MPLS infrastructure is exposed to security attacks
using legitimate IPv4 packets crafted with header options. Further,
LERs that are unable to MPLS encapsulate IPv4 packets with header
options cannot operate as an LER in certain MPLS environments as
described in Section 2.
5. Security Considerations
There are two potential categories of attacks using crafted IPv4
option packets that threaten existing MPLS infrastructures. Both are
described below. To mitigate the risk of these specific attacks, the
ingress LER policy specified above is required.
5.1. IPv4 Option Packets That Bypass MPLS Encapsulation
Given that a router's exception handling process (i.e., CPU,
processor line-card bandwidth, etc.) used for IPv4 header option
processing is often shared with IPv4 control and management protocol
router resources, a flood of IPv4 packets with header options may
adversely affect a router's control and management protocols,
thereby, triggering a denial-of-service (DoS) condition. Note, IPv4
packets with header options may be valid transit IPv4 packets with
legitimate sources and destinations. Hence, a DoS-like condition may
be triggered on downstream transit IPv4 routers that lack protection
mechanisms even in the case of legitimate IPv4 option packets.
IPv4 option packets that belong to a prefix-based FEC yet bypass MPLS
encapsulation at an ingress LER may be inadvertently IPv4 routed
downstream across the MPLS core network (not label switched). This
allows an external attacker the opportunity to maliciously craft
seemingly legitimate IPv4 packets with specific IPv4 header options
in order to intentionally bypass MPLS encapsulation at the MPLS edge
(i.e., ingress LER) and trigger a DoS condition on downstream LSRs.
Some of the specific types of IPv4 option-based security attacks that
may be leveraged against MPLS networks include the following:
o Crafted IPv4 option packets that belong to a prefix-based FEC
yet bypass MPLS encapsulation at an ingress LER may allow an
attacker to DoS downstream LSRs by saturating their software
forwarding paths. By targeting a LSR's exception path, control
and management protocols may be adversely affected and, thereby,
an LSR's availability. This assumes, of course, that downstream
LSRs lack protection mechanisms.
o Crafted IPv4 option packets that belong to a prefix-based FEC
yet bypass MPLS encapsulation at an ingress LER may allow for
IPv4 Time to Live (TTL) expiry-based DoS attacks against
downstream LSRs. MPLS enables IPv4 core hiding whereby transit
IPv4 traffic flows see the MPLS network as a single router hop
[RFC3443]. However, MPLS core hiding does not apply to packets
that bypass MPLS encapsulation and, therefore, IPv4 option
packets may be crafted to expire on downstream LSRs which may
trigger a DoS condition. Bypassing MPLS core hiding is an
additional security consideration since it exposes the network
topology.
o Crafted IPv4 option packets that belong to a prefix-based FEC
yet bypass MPLS encapsulation at an ingress LER may allow for
DoS attacks against downstream LSRs that do not carry the IPv4
routing information required to forward transit IPv4 traffic.
Lack of such IPv4 routing information may prevent legitimate
IPv4 option packets from transiting the MPLS network and,
further, may trigger generation of ICMP destination unreachable
messages, which could lead to a DoS condition. This assumes, of
course, that downstream LSRs lack protection mechanisms and do
not carry the IPv4 routing information required to forward
transit traffic.
o Crafted IPv4 option packets that belong to a prefix-based FEC
yet bypass MPLS encapsulation at an ingress LER may allow an
attacker to bypass LSP Diffserv tunnels [RFC3270] and any
associated MPLS Class of Service (CoS) field [RFC5462] marking
policies at ingress LERs and, thereby, adversely affect (i.e.,
DoS) high-priority traffic classes within the MPLS core.
Further, this could also lead to theft of high-priority services
by unauthorized parties. This assumes, of course, that the
[RFC3270] Pipe model is deployed within the MPLS core.
o Crafted RSVP packets that belong to a prefix-based FEC yet
bypass MPLS encapsulation at an ingress LER may allow an
attacker to build RSVP soft-states [RFC2205] [RFC3209] on
downstream LSRs which could lead to theft of service by
unauthorized parties or to a DoS condition caused by locking up
LSR resources. This assumes, of course, that the MPLS network
is enabled to process RSVP packets.
The security attacks outlined above specifically apply to IPv4 option
packets that belong to a prefix-based FEC yet bypass ingress LER
label stack imposition. Additionally, these attacks only apply to
IPv4 option packets forwarded using the global routing table (i.e.,
IPv4 address family) of a ingress LER. IPv4 option packets
associated with a BGP/MPLS IPv4 VPN service are always MPLS
encapsulated by the ingress LER per [RFC4364] given that packet
forwarding uses a Virtual Forwarding/Routing (VRF) instance.
Therefore, BGP/MPLS IPv4 VPN services are not subject to the threats
outlined above [RFC4381]. Further, IPv6 [RFC2460] makes use of
extension headers not header options and is therefore outside the
scope of this document. A separate security threat that does apply
to both BGP/MPLS IPv4 VPNs and the IPv4 address family makes use of
the Router Alert Label. This is described directly below.
5.2. Router Alert Label Imposition
[RFC3032] defines a Router Alert Label (label value of 1), which is
analogous to the Router Alert IPv4 header option (option value of
148). The MPLS Router Alert Label (when exposed and processed only)
indicates to downstream LSRs to examine these MPLS packets more
closely. MPLS packets with the MPLS Router Alert Label are also
handled as an exception by LSRs and, again, in a less efficient
manner. At the time of this writing, the only legitimate use of the
Router Alert Label is for LSP ping/trace [RFC4379]. Since there is
also no formal standard for Router Alert Label imposition at ingress
LERs:
o Crafted IPv4 packets with specific IPv4 header options (e.g.,
Router Alert) and that belong to a prefix-based FEC may allow an
attacker to force MPLS imposition of the Router Alert Label at
ingress LERs and, thereby, trigger a DoS condition on downstream
LSRs. This assumes, of course, that downstream LSRs lack
protection mechanisms.
6. Acknowledgements
The authors would like to thank Adrian Cepleanu, Bruce Davie,
Rick Huber, Chris Metz, Pradosh Mohapatra, Ashok Narayanan,
Carlos Pignataro, Eric Rosen, Mark Szczesniak, and Yung Yu for
their valuable comments and suggestions.
7. References
7.1. Normative References
[RFC791] Postel, J., "Internet Protocol", STD 5, RFC 791,
September 1981.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC3031] Rosen, E., Viswanathan, A., and R. Callon, "Multiprotocol
Label Switching Architecture", RFC 3031, January 2001.
[RFC3032] Rosen, E., Tappan, D., Fedorkow, G., Rekhter, Y.,
Farinacci, D., Li, T., and A. Conta, "MPLS Label Stack
Encoding", RFC 3032, January 2001.
7.2. Informative References
[RFC2205] Braden, R., Ed., Zhang, L., Berson, S., Herzog, S., and
S. Jamin, "Resource ReSerVation Protocol (RSVP) --
Version 1 Functional Specification", RFC 2205, September
1997.
[RFC2460] Deering, S. and R. Hinden, "Internet Protocol, Version 6
(IPv6) Specification", RFC 2460, December 1998.
[RFC3209] Awduche, D., Berger, L., Gan, D., Li, T., Srinivasan, V.,
and G. Swallow, "RSVP-TE: Extensions to RSVP for LSP
Tunnels", RFC 3209, December 2001.
[RFC3270] Le Faucheur, F., Wu, L., Davie, B., Davari, S., Vaananen,
P., Krishnan, R., Cheval, P., and J. Heinanen, "Multi-
Protocol Label Switching (MPLS) Support of Differentiated
Services", RFC 3270, May 2002.
[RFC3443] Agarwal, P. and B. Akyol, "Time To Live (TTL) Processing
in Multi-Protocol Label Switching (MPLS) Networks", RFC
3443, January 2003.
[RFC4364] Rosen, E. and Y. Rekhter, "BGP/MPLS IP Virtual Private
Networks (VPNs)", RFC 4364, February 2006.
[RFC4379] Kompella, K. and G. Swallow, "Detecting Multi-Protocol
Label Switched (MPLS) Data Plane Failures", RFC 4379,
February 2006.
[RFC4381] Behringer, M., "Analysis of the Security of BGP/MPLS IP
Virtual Private Networks (VPNs)", RFC 4381, February
2006.
[RFC4950] Bonica, R., Gan, D., Tappan, D., and C. Pignataro, "ICMP
Extensions for Multiprotocol Label Switching", RFC 4950,
August 2007.
[IANA] "IP Option Numbers," IANA, February 15, 2007,
<www.iana.org>.
[RFC5462] Andersson, L. and R. Asati, "Multiprotocol Label
Switching (MPLS) Label Stack Entry: "EXP" Field Renamed
to "Traffic Class" Field", RFC 5462, February 2009.
Authors' Addresses
David J. Smith
Cisco Systems
111 Wood Avenue South
Iselin, NJ 08830
EMail: djsmith@cisco.com
John Mullooly
Cisco Systems
111 Wood Avenue South
Iselin, NJ 08830
EMail: jmullool@cisco.com
William Jaeger
AT&T
200 S. Laurel Avenue
Middletown, NJ 07748
EMail: wjaeger@att.com
Tom Scholl
nLayer Communications
209 West Jackson, Suite 700
Chicago, IL 60606
EMail: tscholl@nlayer.net

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